Connected: An Internet Encyclopedia
RFC 792 - Echo or Echo Reply Message

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### Echo or Echo Reply Message

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Identifier          |        Sequence Number        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Data ...
       +-+-+-+-+-

       IP Fields:

       Addresses

          The address of the source in an echo message will be the
          destination of the echo reply message.  To form an echo reply
          message, the source and destination addresses are simply reversed,
          the type code changed to 0, and the checksum recomputed.

       IP Fields:

       Type

          8 for echo message;

          0 for echo reply message.

       Code

          0

       Checksum

          The checksum is the 16-bit ones's complement of the one's
          complement sum of the ICMP message starting with the ICMP Type.
          For computing the checksum , the checksum field should be zero.
          If the total length is odd, the received data is padded with one
          octet of zeros for computing the checksum.  This checksum may be
          replaced in the future.

       Identifier

          If code = 0, an identifier to aid in matching echos and replies,
          may be zero.

       Sequence Number

          If code = 0, a sequence number to aid in matching echos and
          replies, may be zero.

       Description

          The data received in the echo message must be returned in the echo
          reply message.

          The identifier and sequence number may be used by the echo sender
          to aid in matching the replies with the echo requests.  For
          example, the identifier might be used like a port in TCP or UDP to
          identify a session, and the sequence number might be incremented
          on each echo request sent.  The echoer returns these same values
          in the echo reply.

          Code 0 may be received from a gateway or a host.

Next: Timestamp or Timestamp Reply Message

Connected: An Internet Encyclopedia
RFC 792 - Echo or Echo Reply Message